1. Field of the Invention
The present invention relates to a magnetic tape system, more particularly, to a magnetic tape system including a direct reel-to-reel drive means and command prefetch means performing buffered magnetic tape control.
2. Description of the Related Art
In recent years, magnetic tape systems have been developed having reel-to-reel drive means which dispense with the use of vacuum columns and which perform direct control of the reels for winding magnetic tape (for example, see U.S. Pat. No. 4,125,881).
Further, it is known to provide a command prefetch control unit (or buffer adapter) having a buffer memory between the control unit of the magnetic tape system and the host controller so as to prefetch commands and data from the host controller, store the same in the buffer memory, and perform prefetch control for successively executing the commands and data in the buffer memory on the tape drive unit (for example, see U.S. Pat. No. 4,500,965).
This type of magnetic tape system is called a buffered magnetic tape system and enables asynchronous operation of a tape drive unit with respect to commands from a host controller. Therefore, on the host controller side, it is not necessary to wait for the completion of the operation of the tape drive unit for any one command. Commands can be issued continuously. Further, the tape drive unit need not wait for commands from the host controller, but can continuously execute operations. Therefore, the processing efficiency is improved. In particular, the efficiency of operations in the streaming mode in magnetic tape systems of the direct reel-to-reel drive type can be improved.
However, there is a limit to the length of a magnetic tape. If numerous commands are prefetched near the end of the magnetic tape, those prefetched commands cannot all be executed on the tape. Therefore, the command prefetching must be stopped near the end of the magnetic tape. On the other hand, from the point of view of processing efficiency, it is desirable to perform command prefetching as close as possible to the end of the magnetic tape. However, in a magnetic tape system of the direct reel-to-reel type, if the running distance of the magnetic tape is calculated from the diameters of the magnetic tape wound on the two reels, there is the problem that it is not possible to accurately determine when the end of the magnetic tape approaches. The issue now outstanding, therefore, is how to accurately and efficiently stop the command prefetching near the end of the magnetic tape.
Further, if the command prefetching is stopped based on the write data sent from the host controller side, then the problem arises of write operations being performed exceeding the effective length of the magnetic tape in the case of commands not accompanied by write data.